1. Field of the Invention
This invention relates to a metal mask for use in screen printing of a thick film circuit and the like.
2. Description of the Prior Art
Hitherto, there are known three masks which may be employed for screen printing: an emulsion mask for forming a pattern portion by applying a photosensitive resin to a mesh made of tetron, or synthetic fiber stainless steel or the like and then removing resin applied from unwanted portions other than portions exposed according to an exposure process; a metal mask for forming a pattern portion by depositing a metal foil on a stainless steel mesh according to an electroplating process or the like, and then subjecting part of the metal foil to pattern etching; and a solid metal mask for forming a pattern portion by integrally forming a printed pattern layer and an electro-plated layer on a substrate metal foil according to an electroplating process.
In each case of these printing masks, there is provided a mesh layer having rectangular or circular openings, which are arranged orderly, in a pattern opening portion, through which printing paste is to pass, upon printing.
FIG. 2 shows a cross-sectional view illustrative of a condition where printing paste is being transferred by using a prior art metal mask for screen printing. Shown at 1 is a substrate, at 2 a mesh layer, at 4 a substrate to be printed, at 5 paste for thick film printing, and at 6 a squeegee. For printing a thick film circuit on the substrate 4 by using thick film paste 5 and the metal mask, thick film paste 5 is placed on top of the mesh layer 2, and then pressure is applied by the squeeze 6 from above, with squeeze 6 being moved on the screen, so that the thick film paste 5 is printed at the printing pattern 3 on the substrate 4 to be printed, through the meshes of the mesh layer 2. As shown in the drawing, the thick film paste printed on the substrate 4 is difficult to be transferred to the substrate 4 at those portions under the lines or wires of the mesh layer 2, as compared with a case where the paste is transferred through the portions free of the mesh wires, with the result that the upper edges of a printed film in the portion under the lines or wires of the mesh layer 2 exhibit a wave form in its cross section. Particularly, in the case of printing of fine lines, disconnection of lines tends to take place in the edge portions of the printed film. In addition, the thickness of a printed film in a portion positioned under the lines or wires of the mesh layer 2 are reduced, as compared with that of a portion free of the lines or wires, so that there may not be achieved a printed film having a smooth surface.